Coherent gradient sensing ellipsometer

Abstract

Systems and techniques for integrating an optical coherent gradient sensing (CGS) module and another optical sensing module to simultaneously measure the curvature and another property of a specularly reflective surface.

What is claimed is:  
     
       1. A system, comprising: 
       a sample stage configured to hold a sample which has a specularly reflective surface;  
       an input module configured and positioned to produce and direct a substantially collimated coherent input probe beam incident to the reflective surface at an incident angle, optical reflection at the reflective surface producing a reflected probe beam;  
       a beam splitter positioned in an optical path of said reflected probe beam to transmit a portion of said reflected probe beam as a first reflected probe beam and to reflect another portion of said probe beam as a second reflected probe beam;  
       a first detector module positioned to receive said first reflected probe beam and operable to measure curvature of the reflective surface; and  
       a second detector module positioned to receive said second reflected probe beam and operable to measure properties of the sample.  
     
     
       2. The system as in  claim 1 , wherein said first detector module includes two optical gratings disposed process said first reflected probe beam and a first optical detector to receive an output from said two optical gratings. 
     
     
       3. The system as in  claim 2 , wherein said input module is configured so that said input probe beam is polarized, and wherein said second detector module includes a polarization analyzer to receive said second reflected probe beam and an optical sensing array with a plurality of optical sensing pixels to receive transmission of said polarization analyzer. 
     
     
       4. The system as in  claim 3 , further comprising a processing circuit coupled to receive output signals from said first and said second detector modules and operable to process information from said second detector module based on said measured curvature from said first detector module to extract information of the sample at each location that is associated with curvature at said each location. 
     
     
       5. The system as in  claim 3 , further comprising a rotating device engaged to said polarization analyzer to rotate said analyzer. 
     
     
       6. The system as in  claim 3 , wherein said input module includes an optical retardation element operable to produce a time-varying phase retardation on said input probe beam. 
     
     
       7. The system as in  claim 1 , wherein said input module and said second detector module are configured to form an ellipsometer to measure a change in a state of polarization of said input probe beam. 
     
     
       8. The system as in  claim 7 , wherein said ellipsometer is a null ellipsometer. 
     
     
       9. The system as in  claim 7 , wherein said ellipsometer is photometric ellipsometer. 
     
     
       10. The system as in  claim 7 , wherein said first detector module includes: 
       first and second gratings spaced relative to each other in an optical path of said first reflected probe beam and configured to produce a predetermined phase manipulation on a wavefront of said first reflected probe beam;  
       a lens positioned to receive said first reflected probe beam from said first and second gratings and configured to select and combine two diffraction components from said second grating to produce an interference pattern, wherein said selected two diffraction components are produced by said second grating from diffracting two different diffraction components from said first grating and are substantially parallel to each other; and  
       an optical sensing device disposed relative to said lens to receive said interference pattern and produce an electrical signal indicative of said interference pattern.  
     
     
       11. The system as in  claim 10 , further comprising a signal processor connected to receive said electrical signal and configured to extract a spatial gradient information on the wavefront of said reflected probe beam caused by a curvature of the reflective surface, wherein said signal processor determines said curvature of the reflective surface according to said spatial gradient information and processes information from said second detector module based on said curvature to determine a property of the sample. 
     
     
       12. A system, comprising: 
       a light source configured to produce and direct a substantially collimated coherent input probe beam;  
       a sample stage configured to hold a sample which has a specularly reflective surface and positioned in an optical path of said input probe beam, the reflective surface reflecting said input probe beam to produce a reflected probe beam;  
       a polarizer positioned to receive said input probe beam and control a polarization of said input probe beam incident to the reflective surface at an incident angle;  
       a beam splitter positioned in an optical path of said reflected probe beam to transmit a portion of said reflected probe beam as a first reflected probe beam and to reflect another portion of said probe beam as a second reflected probe beam;  
       a first detector module positioned to receive said first reflected probe beam and comprising two spaced optical gratings through each of which said first reflected probe beam is diffracted, and a first optical sensing array to receive a selected optical signal from diffracted signals from said two optical gratings, said first detector module operable to measure curvature of the reflective surface;  
       a second detector module positioned to receive said second reflected probe beam and comprising a polarization analyzer and an optical sensing array, said second detector operable to measure a change in polarization in said reflected probe beam caused by the sample; an optical phase compensator disposed in an optical path between said polarizer and said polarization analyzer in said second detector module to control polarization of said second reflected probe beam incident to said polarization analyzer; and  
       a processing circuit coupled to said first and said second detector modules to determine a property of the sample based on said curvature and said change in polarization.  
     
     
       13. The system as in  claim 12 , wherein said optical phase compensator is configured to produce a time-varying retardation on said second reflected probe beam. 
     
     
       14. The system as in  claim 12 , wherein said polarization analyzer is configured to rotate. 
     
     
       15. The system as in  claim 12 , wherein said first and second gratings are spaced relative to each other in an optical path of said first reflected probe beam and configured to produce a predetermined phase manipulation on a wavefront of said first reflected probe beam, wherein said first detector module further includes: 
       a lens positioned to receive said first reflected probe beam from said first and second gratings and configured to select and combine two diffraction components from said second grating to produce an interference pattern, wherein said selected two diffraction components are produced by said second grating from diffracting two different diffraction components from said first grating and are substantially parallel to each other, and  
       wherein said a first optical sensing array is disposed relative to said lens to receive said interference pattern and produce an electrical signal indicative of said interference pattern.  
     
     
       16. A method, comprising: 
       illuminating a sample having a specularly reflective sample surface with a collimated coherent probe beam to produce a reflected probe beam that has information about said sample;  
       splitting said reflected probe beam into a first portion and a second portion that have said same information;  
       processing said first portion to obtain a curvature of said sample surface from said same information; and  
       processing said second portion to obtain another property of said sample from said same information.  
     
     
       17. The method as in  claim 16 , wherein a change in polarization of said reflected probe beam is measured in processing said second portion. 
     
     
       18. The method as in  claim 17 , wherein said another property of said sample includes a thickness of a film formed said sample surface. 
     
     
       19. The method as in  claim 17 , wherein said another property of said sample includes a refractive index. 
     
     
       20. The method as in  claim 17 , wherein said another property of said sample includes an optical absorption coefficient. 
     
     
       21. The method as in  claim 17 , wherein said another property of said sample includes a stress. 
     
     
       22. The method as in  claim 16 , wherein the processing of said second portion includes: 
       diffracting said second portion by first and second gratings spaced relative to each other in an optical path of said first reflected probe beam to produce a predetermined phase manipulation on a wavefront of said first portion,  
       combining two diffraction components from said second grating to produce an interference pattern, wherein said selected two diffraction components are produced by said second grating from diffracting two different diffraction components from said first grating and are substantially parallel to each other,  
       processing said interference pattern to extract a spatial gradient information on the wavefront of said reflected probe beam caused by a curvature of the reflective surface, and  
       determining said curvature of the reflective surface according to said spatial gradient information.  
     
     
       23. The method as in  claim 16 , wherein said sample surface is a flat mirror, and further comprising using an error in a measurement of said property to determine a degree of collimation of said probe beam. 
     
     
       24. A method, comprising: 
       illuminating a sample having a specularly reflective sample surface with a collimated coherent probe beam to produce a reflected probe beam that has information about said sample;  
       controlling polarization of said probe beam at a desired input polarization prior to incidence upon said sample surface;  
       performing an ellipsometric measurement on a first portion of said reflected probe beam;  
       measuring a second portion of said reflected probe beam to obtain a curvature of said sample surface from said information; and  
       processing said ellisometric measurement by using said curvature to determine a property of said sample.  
     
     
       25. The method as in  claim 24 , wherein said property includes a thickness of a film formed on said sample. 
     
     
       26. The method as in  claim 24 , wherein said property includes a stress on said sample. 
     
     
       27. The method as in  claim 24 , further comprising using said ellipsometric measurement to determine a degree of collimation of said probe beam. 
     
     
       28. The method as in  claim 24 , wherein said curvature is measured by diffracting said second portion of said reflected probe beam through two optical gratings and measuring an interference signal produced by combining two selected diffracted signals. 
     
     
       29. The method as in  claim 24 , further comprising measuring an optical property of said first portion of said reflected probe beam to infer whether a location on said sample surface is concave or convex.